1. Field of the Invention
The invention concerns a bispectral pick-up and false color display system, applicable notably to the sighting and tracking of targets.
For such an application, video recording cameras generally use a silicon sensor that has the advantage of withstanding strong illumination and of having spectral sensitivity that extends from the visible range of the spectrum up to the near infrared range of the spectrum. Experience has shown that the visible range, extending from the 0.4 to 0.7 micron wavelengths, and the near infrared range, extending from the 0.7 to the 1 micron wavelengths, must be used separately for the following reasons.
The fact of crossing the atmosphere on the path between the observed scene and the camera causes light rays to undergo an attenuation and a reduction in their contrast. The attenuation is not the same for all wavelengths, and it depends on atmospheric conditions, namely atmospheric visibility, the nature of the aerosols in suspension in the atmosphere, etc. In many cases, the infrared rays are less attenuated than the visible rays. The reduction in contrast is due to the scattering of light by particles suspended in the atmosphere. The reduction in contrast also depends on atmospheric conditions, and the visible and infrared rays are not affected by the same reduction in contrast.
The scene observed is itself different depending on whether it is observed in the infrared range or in the visible range. The contrast between a target and the background of the picture may be different in these two ranges or regions of the spectrum. If, as is very common, the background consists of vegetation, then the contrast consistently bears the opposite sign in these two ranges. For example, a concrete structure surrounded by grass will appear as being bright against a dark background in the visible range, and it will appear as being dark against a bright background in the infrared range.
However, observation in the visible range of the spectrum and observation in the infrared range are each useful. Depending on the atmospheric conditions, and on the scene observed, it will be more useful to observe the scene in one range or in the other.
Because of the reversal of contrast, in the vicinity of the 0.7 wavelength, one and the same sensor cannot be used for both these regions of the spectrum, otherwise, the picture obtained has a contrast which is the mean of the contrasts in the two ranges, i.e. a very low contrast. A silicon sensor therefore cannot be used in its entire range of sensitivity.
2. Description of the Prior Art
In a standard way, the video cameras used in weapons systems are fitted out with a switch-over filtering device. The operator has a hand-operated control which can be used to filter either the visible rays or the infrared rays. The operator makes this choice as a function of the picture that he observes on the screen of a monochrome monitor, in seeking the best contrast. In practice, the operator does not always have the time nor is it possible for him to review the choice that he has made a few minutes before. However, if the distance between the camera and the target has changed, the maximum contrast may no longer be in the spectral field that was chosen. At a long distance, the attenuation of the light beams and the degradation of their contrast by the scattered light are more moderate in the infrared range. By contrast, if the camera approaches the target, it can happen that the contrast becomes greater in the visible range.
When the distance varies swiftly, it is complicated to change the filtering. In standard systems, the change in the filtering causes a brief loss of pictures which may possibly cause an automatic target-tracking operation, being performed by this camera, to get unlocked. In practice, the user does not take this risk and therefore keeps the same filtering mode.
To use the visible range and the infrared range simultaneously, it is possible to envisage the making of a system that has a bispectral camera to analyze a picture in the visible range and a picture in the infrared range, and has two monochrome monitors placed side by side. However, this system is too slow and too tiring for the operator.
It is also possible to envisage the use of a single monochrome monitor receiving a video signal obtained by the linear combination of the two video signals given by the camera. Experience shows that the displayed picture is flat, for the contrasts in intensity in the two ranges compensate for each other more or less, at the instant when the signals are combined.